President Diaz Ordaz, in a speech from the city of Guadalajara, tendered his famous “outstretched hand” to the students in exchange for their submission. An outstretched hand — if you apologize.
The imaginative riposte appeared the next day on thousands of handbills and flyposters: A la mano tendida, la prueba de la parafina (Give the outstretched hand the paraffin test).
definition (courtesy the Oxford English Dictionary online):
[probably after American Spanish; use of the test in Mexico City is reported from 1931] a forensic test to indicate whether a person has recently fired a gun, in which the person's hand is coated in hot paraffin wax which cools and sets and is peeled off and tested for the presence of residue from the gun.
source: ’68 a memoir by Paco Ignacio Taibo II, translated by Donald Nicholson-Smith